Wells drilled to produce petroleum or gas products typically also produce brine water which, often, is near or at sodium chloride saturation levels. It is not atypical for salt crystals in the brine to form a “salt block” in the well and/or flow lines into or out of the well. Salt blocks may form by the cooling of the water as oil or gas leaves the well. In gas wells, they may also be formed as the concentration of the brine increases downhole during the stripping of water vapor by the gas.
Historically, salt blocks and deposits have been treated by circulating fresh water into the well. While usually effective at dissolving the salt, this treatment can be expensive and inconvenient. In addition, fresh water may not be readily accessible at the drilling site, necessitating the transport of water over potentially long distances. Such water treatment may further require production from the well to be significantly reduced or stopped.
Salt inhibitors typically function by binding to the surfaces of salt crystals, preventing further growth of the crystal. This effectively increases the solubility of the salt in solution, as it is incapable of precipitating. Other mechanisms of action may be possible, resulting in an increased solubility of salt in solution.
Various salt inhibitors have been discussed in the literature. For instance, SPE 10097 (Society of Petroleum Engineers of AIME, Oct. 5-7, 1961) describes the evaluation of five commercially available salt inhibitors. Laboratory assays were performed to determine effectiveness in maintaining salt saturation at downhole temperatures. Tests were also performed in a Williston Basin drilling program. The chemical compositions of the inhibitors were not disclosed. U.S. Pat. No. 3,213,018 suggests the use of iron or cobalt salts as a method to inhibit deposition of sodium chloride from saturated solutions. U.S. Pat. No. 7,028,776 discloses the use of nitroliotriacetamide and its salts as a sodium chloride salt block inhibitor. In certain circumstances, the effectiveness of certain sodium chloride salt block inhibitors of the prior art, in the presence of iron ions, was weakened. Further, since such sodium chloride salt block inhibitors were generally not biodegradable, their use in offshore applications was restricted.
Alternative salt block inhibitors have been sought which do not significantly reduce production of oil or gas from producing wells, do not lose effectiveness in the presence of iron ions and are biodegradable.